Donor–acceptor dichotomy in novel Schiff bases: comprehensive spectroscopic and DFT investigation of intramolecular hydrogen bonding and charge-transfer properties

Abstract

Intramolecular charge-transfer (CT) and hydrogen bonding in Schiff bases underpin their utility as photoactive and sensing materials, yet quantitative structure–property relationships that integrate solvatochromism with electronic-structure descriptors remain limited. Here, five strategically designed Schiff base ligands derived from 1,3-diaminopropane are used to isolate donor/acceptor effects, substitution pattern, and planarity on CT behaviour. UV-vis spectra at fixed concentration (1 × 10⁻⁵ M) in polar protic and aprotic solvents reveal systematic CT band shifts (360–420 nm), while cyclohexane data are used qualitatively as a low-polarity reference due to limited solubility. Multiparametric Kamlet–Taft analysis yields statistically significant correlations for hydroxyl derivatives (R² ≥ 0.94; dominant π* and α terms with p < 0.05), whereas nitro analogues show weaker fits and non-significant α contributions, consistent with the absence of phenolic hydrogen bonding and, for the 2-nitro isomer, steric disruption of planarity. Quantitative comparison of NBO second-order perturbation energies (7.8–14.7 kcal mol⁻¹), ¹H NMR shifts (δ(OH) = 13.5 ppm), and AIM electron densities establishes a robust link between computed hydrogen-bond strength and experimental resonance-assisted hydrogen bonding signatures. DFT geometries reveal that increasing Ar–CN–CH torsion (2.5–12.3°) enlarges the HOMO–LUMO gap (3.14–3.22 eV) and blue-shifts CT bands. Comparison with literature shows these values are typical, while the integrated multiparametric solvatochromism–NBO–AIM approach provides a quantitative, transferable framework for predicting CT behaviour in donor–acceptor Schiff bases.